This invention relates generally to television high voltage systems and particularly to shutdown circuits used therein.
In a typical color television receiver scansion and display system, a cathode ray tube (CRT) display device includes a trio of electron beam sources which are directed at a tri-color phosphor viewing screen. Horizontal and vertical scansion circuitry within the receiver locally generate scansion signals which are synchronized to reference information within the received signal. The scansion signals are applied to an electromagnetic deflection yoke positioned on the envelope of the CRT producing vertical and horizontal scansion of the viewing screen. The cathode ray tube requires a number of operating potentials the highest of which is an accelerating potential of approximately 25 to 30 kilovolts which is generally referred to as the high voltage. This potential is applied to an electrode within the CRT to accelerate the electrons within the directed beams to an energy level sufficient to cause light emission by impacted phosphor areas and illumination of the viewing screen.
In the great majority of television receivers the horizontal scansion system produces this high voltage in addition to the horizontal scansion signals. Horizontal scansion includes a relatively slow scan deflection of the electron beams followed by a relatively fast retrace deflection in which the beams are deflected back to the "start" of scan position. The scansion signal producing this retrace comprises a short duration high amplitude pulse which is also used to generate CRT high voltage. Most receivers use a tertiary winding on the horizontal transformer together with a rectifier or voltage multiplier to raise the voltage to sufficient level for high voltage production. In the former system the tertiary transformer winding is rectified directly while in the latter a familiar capacitor diode matrix is used to boost the voltage and rectify.
It is generally desirable for purposes of picture sharpness, brightness and color rendition to maintain a relatively high accelerating potential. However, cathode ray tubes have a tendency to produce prohibitive amounts of radiation when excessive high voltage is used. As a result care must be taken to assure that the accelerating potential does not exceed the radiation producing threshold.
It is well known to employ high voltage shutdown circuitry which monitors the accelerating potential or some related voltage and disables or reduces the accelerating potential in the event of excess output. Such circuitry may include threshold circuitry detecting either the high voltage directly or a voltage derived such as that used for CRT focus. Another alternative is to use circuitry detecting the peak or average voltage of the retrace portion of the horizontal scansion signal which, of course, varies in a predictable relationship with CRT high voltage. In either case, the most typical operation provides complete shutdown of the high voltage system in the event of an excess.
In addition to problems of prohibitive radiation produced by excessive high voltage, cathode ray tubes are susceptible to damage by high currents in conjunction with otherwise unobjectionable accelerating potential levels. The energy with which the accelerated electrons impact the CRT parallax barrier as well as the viewing screen is determined largely by the high voltage but the total energy imparted is, of course, also dependent on the number of impacting electrons (e.g., beam current). High beam currents generate heat which if not dissipated may cause damage to the cathode ray tube itself. For example, the parallax barrier may be overheated or the viewing screen phosphors may be burned. Also excessive locallized heat within the tube may produce fracture of the CRT envelope which, of course, usually renders the tube useless.
For these and other reasons most television receivers include beam current limiting circuitry which functions to minimize or avoid prohibitively high beam current. Such circuits are nearly endless in variety but all can be said to perform the common functions of somehow detecting beam current and acting upon signal processing circuitry (which controls beam current) in a negative feedback manner. Such circuits perform satisfactorily under most conditions but frequently do not provide affirmative protection against the types of failures described above and in some cases may themselves have failure modes which result in production of excessive beam current.
The problems of overdissipation and radiation production of the cathode ray tube in modern television receivers are made more difficult by the improvements in regulation of operating supply and high voltage generating circuitry. Modern circuitry is able to sustain great overloads and still maintain high voltage output. Because these improved systems are capable of producing greater power levels they are not in any real sense "self-limiting". Therefore, while providing considerable advantages in picture quality and other performance criteria, such "stronger" high voltage supplies also have a greater capacity for causing CRT damage in the above-described failure modes.